Next, we took the resistivity measurements under magnetic field \(\mathbf {H}\)  perpendicular to and parallel with \(\mathbf {b}\) , and the results are displayed in Fig. REF (b) and (c), respectively. For \(\mathbf {H}\perp \mathbf {b}\) , magnetic field gradually suppresses the SC transition, and when \(\mu _0H=0.12\)  T, seldom resistivity drop can be seen. The upper critical field \(\mu _0H_{c2}(0)\)  can be obtained from the \(\rho _b(H, T)\)  data, by fitting the field dependent \(T_c^{on}\)  to the Ginzburg-Landau formula[1]}, \(\mu _0H_{c2}(T)=\mu _0H_{c2}(0)(1-t^2)/(1+t^2)\) , where \(t=T/T_c^{on}\)  is the reduced temperature. This fit results in \(\mu _0H_{c2}^{\perp b}(0)=0.13\)  T. The measurements for \(\mathbf {H}\parallel \mathbf {b}\)  were made on another sample whose \(T_c^{on}\)  is a little higher, but we notice that a second step is visible between 0.4 and 0.8 K before \(\rho _b\)  finally drops to \(\sim \) 0. This is probably because of phase separation or some weak links between superconducting granules. In this field orientation, we derived \(\mu _0H_{c2}^{\parallel b}(0)=0.30\)  T. The anisotropy of upper critical field thus is \(\sim \)  2.3. These \(\mu _0H_{c2}\)  values are far less than the Pauli paramagnetic limit \(\mu _0H_P=1.84 T_c\approx 1.4\)  T [2]}, [3]}, implying that a spin-triplet Cooper pairing is very unlikely.
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